1. Field of the Invention
The present disclosure relates to an optical read/write apparatus that reads, writes or erases information from/on an optical storage medium such as an optical tape, an optical disc or an optical card. More particularly, the present disclosure relates to an apparatus that carries out verification on signal marks being recorded on an optical storage medium while writing data on it.
2. Description of the Related Art
Recently, the size of digital data to process has been rising steeply year by year as the resolutions of video data and still picture data have been tremendously increased and as increasing numbers of paper media have been converted into electronic ones. Meanwhile, so-called “crowd computing” technologies that allow people to use various kinds of applications and services via servers and storage systems on some network have become more and more popular nowadays. According to such crowd computing technologies, as a huge number of users save various kinds of data on that storage system on the network, the amount of data accumulated there should keep on skyrocketing from now on.
In the meantime, as regulations have been established one after another with regard to the duty of preserving such a huge amount of data saved, it should also be increasingly important to devise a method for saving that enormous amount of data as securely and as reliably as possible. An apparatus that writes data of such a huge size optically on a storage medium can perform the operation of seeing if (signal) marks have been recorded just as intended on the optical storage medium in order to increase the reliability of writing. Such an operation will be referred to herein as a “verify operation”.
A known apparatus that performs a read/write operation on a write-once or rewritable storage medium such as an optical disc reads data that has been just written and compares the data that has been read to the data to write in order to detect an error, if any, lest the write operation should fail due to a defect on the storage medium.
Such a verify operation is often performed in a unit at which a constant write or transfer rate can be maintained, not after everything has been written. That is to say, every time the disc has turned to a predetermined degree, the write operation is suspended, a track jump is made to return to the previous location, that portion on which data has just been written is scanned to detect any error, and then a track jump is made once again to move to a different area and write the next data there. And this series of operations is carried out over and over again. That is why although reliability can be certainly ensured in this way for the data that has been written, it takes a longer time to get the write operation done.
If any error is detected when the data that has just been written is read, then the write operation is retried on another area, not the area on which the write error has occurred. On an optical disc, a set of data and its ID information are stored on the basis of a unit area called a “sector”. Thus, the data that has been written with an error on a sector is rewritten on another sector (which will be referred to herein as a “replacement sector”).
A known read/write apparatus that makes such data correction is disclosed in Japanese Laid-Open Patent Publication No. 59-113509 (which will be referred to herein as “Patent Document No. 1” for convenience sake), for example.
Lately, as candidate read/write apparatuses that can save and archive data for a long time in order to meet the rising demand for storing a huge size of data with as high reliability as possible, proposed are an apparatus that uses a so-called “optical tape”, which is a kind of an optical storage medium in a tape shape, and an apparatus that handles a combination of multiple optical disc drives at the same time. Such a read/write apparatus to process a huge size of data should not only write and transfer data at sufficiently high rates but also keep the reliability of the written data as high as possible.
Nevertheless, for a storage medium with a low degree of random accessibility such as the tape medium, it is difficult to increase the write rate as long as such a method of writing data and checking the data just written time-sequentially over and over again is adopted as in the known optical disc drive described above.
Thus, to meet such a demand, a so-called “DRAW (direct read after write)” technique for performing a write operation and a read operation for verification purposes at the same time has been proposed.
A known read/write apparatus that adopts such a DRAW technique is disclosed in Japanese Laid-Open Patent Publication No. 63-249941 (which will be referred to herein as “Patent Document No. 2” for convenience sake), for example. FIGS. 27A through 27C illustrate an exemplary arrangement and operation of an optical pickup as disclosed in Patent Document No. 2.
As shown in FIG. 27A, the optical system of this optical pickup includes a light source 410, a diffractive element 411, a polarization beam splitter 403, a wave plate 404, a collimator lens 405, a mirror 406, an objective lens 407, a detector lens 402, and a photodetector 401. The light emitted from the light source 410 gets diffracted by the diffractive element 411 and split mainly into a zero-order light beam and ±first-order light beams, which are then condensed by the objective lens 407, thereby forming three condensed beam spots (that are a main spot and two sub-spots) on the same track on the optical storage medium 408.
FIG. 27B illustrates the arrangement of light beam spots that are formed on the surface of the optical storage medium 408.
In the example illustrated in FIG. 27B, the main spot 500 formed by the zero-order light beam is a write beam spot, which is used to write a signal on the storage medium. On the other hand, the two sub-spots 510 and 520 formed by the ±first-order light beams are read beam spots, which are used to read the written signal. Due to the efficiency ratio of the diffraction grating, the intensities of the first-order light beams are much lower than that of the zero-order light beam. That is why the signal that has been written is never erased or altered by the two sub-spots 510, 520.
The main spot 500 and the sub-spots 510 and 520 are located on the same track. And these spots move on the storage medium in the direction indicated by the arrow a. Such movement of the main spot on a track of a storage medium will be referred to herein as “scanning the storage medium with a write beam”. In the same way, such movement of the sub-spots on a track of a storage medium will be referred to herein as “scanning the storage medium with a read beam”. In performing a DRAW operation, the same location on an optical storage medium is scanned with a write beam before being scanned with a read beam. More specifically, of these two sub-spots, the sub-spot 510 moves behind the write spot to read the recorded mark. Meanwhile, the other sub-spot 520 moves ahead of the write spot, and its reflected light includes no information about the recorded mark. These light beams are reflected from the optical storage medium 408, transmitted through the optical system, and then incident on the photodetector 401, which detects their quantities of light.
FIG. 27C illustrates the arrangement of photodiodes in the photodetector 401.
The main quadruple photodiode 121 shown in FIG. 27C receives the zero-order light beam (i.e., the reflected light that has left the main spot). The magnitude of astigmatism produced by the detector lens 402 shown in FIG. 27A changes with the degree of defocusing, thereby detecting a focus signal. The main photodiode 121 also detects a tracking error signal by the push-pull method. On the other hand, the sub-photodiodes 122 and 123 receive reflected light that has left the sub-spot 510 and reflected light that has left the sub-spot 520, respectively.
The light source 410 emits a light beam that has been modulated with a modulation signal in order to form information pits on the optical storage medium 408.
Naturally, the read beams that have been emitted from the same light source 410 have also gone through that modulation. That is why the reflected light of the sub-spot 510 that moves behind the write spot in the two read spots of the ±first-order light has a signal component, to which a variation in reflectance caused by a recorded mark and a variation in the quantity of light due to the modulation of light have been added. Meanwhile, the other sub-spot 520 moves ahead of the main spot 500 through an unrecorded portion, and therefore, its reflected light has not been affected by the variation in reflectance caused by the recorded mark. Consequently, only a signal representing a variation in the quantity of light due to the modulation of the light by the light source is obtained from the reflected light of the sub-spot 520 that moves ahead of the main spot 500. For that reason, by performing a differential arithmetic operation on the two signals representing those two sub-beams, a read signal (i.e., a monitor signal for verification purposes) can be obtained.
By adopting the DRAW technique for forming the write spot (i.e., the main spot 500) and the read spots (i.e., the sub-spots 510 and 520) at the same time and for reading a signal that has just been written while performing a write operation, a system that achieves even higher write and transfer rate and ensures a good deal of reliability is realized.
As already described as an example of the background art, an optical read/write apparatus that can get a DRAW operation done easily at a low cost may use a configuration for splitting the light that has been emitted from a single light source into a write beam and read beams. If such a configuration is adopted, however, a write modulation signal is also superposed on the read beams, and therefore, the write modulation component needs to be removed from the signal obtained based on the read beams. It is theoretically possible to remove the write modulation component by such a technique as what is used in the related art. However, while a write operation is being performed, the amplitude of the write modulation component in a sub-beam signal is actually much greater than that of a signal representing a recorded mark component. That is to say, although the dynamic range of the circuit needs to be set to prevent the sub-beam signal from getting saturated with the write modulation component, the signal component of the recorded mark component obtained in such a situation becomes very small. In addition, it is very difficult to remove the write modulation component perfectly due to a subtle differential balance to be struck when the write modulation component is obtained by calculating the difference from another sub-beam signal or a difference in frequency characteristic or delay between the signals. For these reasons, a highly sophisticated signal processing system should be used to keep the SNR of the DRAW signal high enough.
Thus, an embodiment of the present disclosure provides an optical read/write apparatus that can get the DRAW operation done with good stability.